The present invention relates to lithium-iron oxide particles and a process for producing the lithium-iron oxide particles, and more particularly to lithium-iron oxide particles suitable as a cathode active material used in lithium ion batteries, which particles have a corrugated layer structure and a high crystallinity, and are excellent in electrochemical reversibility, and a process for producing such lithium-iron oxide particles.
In recent years, in association with developments of personal computers and portable equipment such as portable phones, there have been an increasing demand for batteries as a power source therefor. Especially, a lot of studies for lithium ion batteries have been made in various fields because lithium is expected to provide high energy density batteries having a light weight due to a small atomic weight of lithium and a high electromotive force due to a high ionization energy of lithium.
In addition, recently, as a cathode active material usable in lithium ion batteries, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2 or the like have been earnestly studied because these substances can generate a high voltage. Among them, Li.sub.x CoO.sub.2 has been already put to practical use. However, since the Co- or Ni-containing compounds such as Li.sub.x CoO.sub.2 or Li.sub.x NiO.sub.2 are expensive and Co or Ni are elements which are relatively less in production, the use of these compounds as a material for the as a cathode active material is not economical.
For this reason, as an economical material for as a cathode active materials, attention has been paid to lithium compound particles which have such a structure that Co or Ni of the afore-mentioned Co- or Ni-containing compounds is replaced with other transition elements, that is, lithium-iron compound particles, because Fe is not only inexpensive but also readily and extensively available.
Further, in the case of lithium ion batteries using lithium-iron oxide, lithium ions are repeatedly electrochemically introduced into and removed from ion sites in the lithium-iron oxide. However, when the lithium-iron oxide suffers from change in crystal structure, ion sites capable of receiving lithium ions or conduction paths for lithium ions in the crystal are apt to be dissipated, thereby causing deterioration in electrochemical reversibility of the lithium ion battery. Therefore, it is strongly demanded to enhance a crystallinity of the material for as a cathode active material.
Li.sub.x CoO.sub.2 or Li.sub.x NiO.sub.2 known as a material for as a cathode active material have a layered rock salt-type (.alpha.-NaFeO.sub.2 type) crystal structure. As other compounds than Li.sub.x CO.sub.2 and Li.sub.x NiO.sub.2 which have such a layered rock salt-type crystal structure, only Li.sub.x VO.sub.2 and Li.sub.x CrO.sub.2 have been known at present.
In the case of a high temperature-synthesizing process which process comprises calcining mixed particles of iron oxide and a lithium compound at a temperature of about 800.degree. C., the obtained lithium-iron compound particles have a disordered tetragonal rock salt-type crystal structure. On the other hands, in the case of a low temperature-synthesizing process which comprises calcining mixed particles of iron oxide and a lithium compound at a temperature of about 400.degree. C. to 500.degree. C., the obtained lithium-iron compound particles have an ordered tetragonal crystal structure. However, any of the thus-produced lithium-iron compound particles could not act as a cathode active material for lithium ion batteries.
One of the present inventors has already produced, as a material for as a cathode active material usable in lithium ion batteries, lithium-iron oxide particles represented by the formula of Li.sub.x FeO.sub.2, which have a corrugated layer structure similar to the crystal structure of known Li.sub.x MnO.sub.2 ("Proceeding of the 36th Battery Symp. in Japan", pages 23-24, 1995).
However, the Li.sub.x FeO.sub.2 particles having a corrugated layer structure is unsatisfactory in crystallinity thereof, thereby causing a problem that an electrochemical reversibility of the particles is deteriorated.
The problem concerning the deterioration in electrochemical reversibility is explained in derail below.
The Li.sub.x FeO.sub.2 having a corrugated layer structure is formed by an ion exchange reaction between protons contained in .gamma.-FeOOH and lithium ions. When the ion exchange reaction is conducted at an elevated temperature, .alpha.-Li.sub.x FeO.sub.2 is improperly produced as a high temperature-stable phase. Therefore, it is necessary to conduct the ion exchange reaction at a temperature as low as not more than 350.degree. C. However, when the ion exchange reaction is conducted at such a low temperature, the obtained Li.sub.x FeO.sub.2 has a low crystallinity and, therefore, the crystal structure thereof tend to become unstable. If lithium ions are repeatedly electrochemically introduced into and removed from the ion sites in Li.sub.x FeO.sub.2 having such an unstable crystal structure, the crystal structure located in proximity of the ion sites undergoes unsuitable change, so that there arises the afore-mentioned problem that the electrochemical reversibility of Li.sub.x FeO.sub.2 is deteriorated.
In addition, the Li.sub.x FeO.sub.2 particles having a corrugated layer structure have a low electronic conductivity and a small diffusion coefficient of lithium ions between the respective layers thereof. In the case where the Li.sub.x FeO.sub.2 particles are used as an electrode material for a lithium ion battery, there is caused a problem that the electrode reaction speed in the battery is low, so that the electric current operated by the battery becomes small.
On the other hand, at present, the demanded materials for as a cathode active materials are lithium-iron oxide particles which can show a good stability with the passage of time and can be readily handled. However, the afore-mentioned lithium-iron oxide particles having a corrugated layer structure tend to undergo unsuitable change in crystal structure with the passage of time so as to be transformed into lepidocrocite, thereby also causing a problem that a satisfactory activity required as a cathode active material for lithium ion battery cannot be attained.
For this reason, when such lithium-iron oxide particles are used as a material for as a cathode active material used in lithium ion batteries, sufficient care must be taken upon handling thereof.
As a result of earnest studies by the present inventors, it has been found that by heating a mixture of lepidocrocite particles containing at least one metal selected from the group consisting of cobalt, nickel, manganese and aluminum, and a lithium compound at a temperature of 100 to 150.degree. C., lithium-iron oxide particles produced have a corrugated layer crystal structure and an excellent electrochemical reversibility, and are suitable as a cathode active material used in lithium ion batteries. The present invention has been accomplished on the basis of this finding.